RNA aptamers have been generated against a variety of proteins including a few cell surface receptors. In general such aptamer act as potent inhibitors of their target proteins both in vitro and in vivo. Moreover, a few of these antagonistic aptamers have been evaluated in clinical trials including one we developed against factor IXa and one that has been approved by the FDA for treating age related macular degeneration. However until last year, no aptamer has been described that can act as an agonist. In this proposal we explore the ability of aptamers to activate cell surface receptors. In the preliminary results section we describe a series of studies demonstrating that dimeric versions of aptamers that we have made against two different receptors on T-cells, 4-IBB and 0X40, are able to activate these receptors on primary murine T cells resulting in T-cell proliferation, cytokine release and enhanced antitumor vaccine activity in murine tumor immunotherapy models. To our knowledge these are the first two aptamers that have been identified that can act as agonists. Here we are seeking support to evaluate the activity of these two agonistic aptamers as well as determine whether a third aptamer that we have recently made against stem cell factor receptor, c-Kit, can also act as an agonist and activate stem cell factor receptor and erythropoiesis. Analysis of these aptamers will include assessing their ability to deliver siRNAs to primary 1-cells and T-celi lymphoma and leukemia cell lines (0X40 and 4-1 BB aptamers) and erythrocyte precursors and c-Kit positive tumor cells (cKit aptamer). Technologies that mediate targeted delivery of small interfering RNAs (siRNAs) are needed to improve the therapeutic efficacy, safety and cost effectiveness of siRNA-based therapeutic agents